Wildfires have the potential to dramatically alter forest carbon (C) storage and nitrogen (N) availability, but the long-term biogeochemical legacy of these events is poorly understood. I used a high-resolution lake-sediment record of fire occurrence and biogeochemical change from a subalpine watershed in Colorado, USA, to examine the nature, magnitude, and duration of fire-induced ecosystem impacts over the last ca. 4250 yr. Superposed Epoch Analysis (SEA) revealed pronounced biogeochemical responses to multiple high-severity fires, inferred from statistically significant peaks in charcoal accumulation and magnetic susceptibility (an indicator of erosion). On average, fires were followed closely by significant increases in bulk sediment N isotopic composition (d15N) and bulk density, and declines in %C and %N - likely reflecting destruction of the forest floor, terrestrial C and N losses, and erosion. Anomalously low sediment C:N ca. 20-50 yr after fires suggests a long-lived reduction in terrestrial organic matter subsidies to the lake. The magnitude of post-fire change was well-correlated with charcoal peak magnitude, indicating that the extent of disturbance impacts scaled directly with inferred fire size and/or severity. Trends ca. 30-75 yr following fire, including a significant decline in d15N, were consistent with patterns observed in chronosequences of forest C and N accumulation, suggesting that terrestrial successional processes were reflected in the sediments. The results of this study, the first to systematically test the utility of lake-sediment d15N as an indicator of fire-induced N cycle change, indicate that high-resolution analysis of sediment records may offer a unique and powerful tool for elucidating the effects of fire on ecosystem biogeochemistry over decades to millennia.

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